We and others have shown that bone marrow derived cells can take on the gene expression pattern and phenotype of alveolar type II pneumocytes. The mechanism(s) for this engraftment is(are) not yet known. One confirmed mechanism is fusion of marrow derived cells with mature non-hematopoietic cells under conditions of severe cellular injury. Using recipient mice whose pulmonary function is compromised due to knockout of surfactant protein C, we have confirmed the ability of bone marrow transplantation to restore type II pneumocytes that produce surfactant protein C in the lungs. In this application we propose to determine which BM subpopulations(s) become epithelial cells as well as the underlying mechanisms responsible. We will test the hypothesis that different BM subpopulations are capable of engraftment as type II pneumocytes, and that different degrees of tissue injury may promote engraftment of marrow derived epithelial cells via different mechanisms. Using the surfactant protein C knock-out model, we will compare the degree and kinetics of engraftment of type II pneumocytes from different BM subpopulations, assess the degree to which fusion is involved using Cre-lox and dual reporter mouse strains. These studies are critical for obtaining a better understanding of how BM derived cells in the adult take on the gene expression pattern of mature epithelial cells. The discoveries made could lead to novel therapeutic approaches for repair of tissues damaged by injury or disease. Project Narrative Studies to determine the mechanisms by which bone marrow derived cells engraft as functional epithelial cells in tissues after injury are important because this engraftment may represent a renewable source of stem cells that could be exploited for new cell- based therapies for tissue repair. The work proposed addresses these important issues in the setting of lung injury. This is an important phenomenon and if it could be harnessed for various clinical applications would be of potentially enormous benefit. Further, the cells could be used vehicles for gene therapy to treat pediatric lung diseases such as surfactant protein deficiency or alpha-1 anti-trypsin deficiency.